Embodiments of the present invention pertain generally to implantable medical devices, and more particularly pertain to implantable and external medical devices that utilize ST segment variations within cardiac signals to record and present coronary burden information to illustrate trends.
An implantable medical device is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm.
Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. Ischemia arises during angina, acute myocardial infarction, coronary angioplasty, and any other condition that compromises blood flow to a region of tissue. When blockage of an artery is sufficiently severe, the cardiac ischemia becomes an acute myocardial infarction (“AMI”) or also referred to as a myocardial infarction (“MI”) or a heart attack. Ischemia and AMI represent related types of acute coronary burden.
Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Electrocardiograms (“ECG”) and intra-cardiac electrograms (“IEGM”) are two types of cardiac signals that are very useful for diagnosing ischemia and locating damaged areas within the heart. Both ECGs and IEGMs comprise various waves and segments that represent heart depolarization and repolarization. The ST segment represents the portion of the cardiac signal between ventricular depolarization and ventricular repolarization. Variation, for example, elevation or depression in the voltage level (e.g., measured in millivolts, ADC counts or a percentage of ST shift, and the like) of the ST segment in an ECG or IEGM may result when there are abnormalities in the polarization of cardiac tissue during an acute coronary episode such as an ischemia or an acute myocardial infraction (AMI).
Techniques have been developed for detecting cardiac ischemia using implanted medical devices. However, coronary episodes (e.g. ischemias) may occur sporadically occurring sometimes once in a few weeks or other times occurring many times in a day. Today, external programmers exist that communicate wirelessly or via telemetry with the IMD and download, from the IMD, a limited amount of information related to a coronary event. For example, conventional programmers notify a physician that a coronary event has occurred and the time at which the event started. The programmers also allow a physician to view the IEGM signal at the onset of the coronary event. Today, IMDs process coronary events as a triggering type event that directs the IMD to take an action such as to save, in long-term memory, the IEGM signal as well as other information such as patient status associated with the event.
However, conventional approaches to ischemic events have not satisfactorily captured the various types of information that characterize an ischemic episode. Nor have conventional approaches been able to record and present long-term ischemia related burden information and the trends associated therewith. A need remains for improved methods and systems for trending acute coronary burden based on ST segment variations.